setup a proper build infrastructure for win32 builds

Developing on win32 is a nightmare and we rely on far too many sources for all the DLLs we need.
We should setup a cross compile environment based on mingw so that anyone can just follow some simple (?) steps and obtain all the components, built from source, with or without debugging symbols, etc
Including:

gtk

gstreamer

x264

libav

webp

etc..
Then we can also automate the build and include that from the buildbot.

This should make the code more reliable/maintainable and may well help in solving a number of ugly win32 tickets:

Maybe we can mix cross-compilation and native versions?
Or maybe we should start considering moving to Python 3 + native clients?, as this may well be easier to build with a cross compiler (see ​python-mingw)

OK, here are a few things that do build. Loosely based on the list of dependencies found ​here for OSX builds.
Unless otherwise stated, just run mingw32-configure && make && make install in the source:

Which fails to find Python.h... and adding it by hand for now only fails a little bit later... Copying all the headers to the source directory is easier than fighting wine / unix paths, but it eventually still fails.

So even with this one, we will have to try to rebuild a fair amount of stuff.
There are some newer builds here: ​http://win32builder.gnome.org/, but no idea how well they work and how official they are... what a mess.

I think building pygtk2 from source will just be too hard (cross compiling is a no go because of all the extensions that need to be compiled - already hard enough even without the cross compiling part), especially if we need to update some of the libraries for security reasons (ie: libpng.

So I am closing this as WONTFIX for now, hoping that we can move win32 clients to GTK3 (#90).